Electrolysis: Obtaining hydrogen from water:
The Basis for a Solar-Hydrogen Economy

This project involves a fascinating experiment in
electrochemistry that illustrates several important energy related processes,
and provides an ideal context for discussion of several issues related to
electricity generation.

Introduction to Electrolysis: Hydrogen from
Water

As covered in the discussion section below, it is
possible to use hydrogen as a fuel, that is, a way to store energy, for days
when the Sun doesn't shine, or at night time, or for powered mobile devices such
as cars.

The process by which we generate hydrogen (and
oxygen) from water is called electrolysis. The word "lysis"
means to dissolve or break apart, so the word "electrolysis" literally
means to break something apart (in this case water) using electricity.

Electrolysis is very simple - all you have to do is
arrange for electricity to pass through some water between to electrodes placed
in the water, as shown in the diagram above. Its as simple as that! The
principle of electrolysis was first formulated by Michael Faraday in 1820.

If the electricity used for electrolysis is
generated from fossil fuels, then carbon dioxide would be emitted in support of
our electrolysis process, and the advantage of using hydrogen as a fuel would be
lost. But if the electricity is produced by solar cells, as we suggest in
the diagram above, then there will be no pollutants released by our process.

Materials you will need

A battery or solar panel with a voltage greater
than 1.5 volts - 9 volt batteries work well.

Two pieces of electrical wire about a foot long.
Its convenient, but not necessary, if the wire have alligator clips at each
end.

Two number 2 pencils

A jar full of tap water

small piece of cardboard

electrical or masking tape.

Tools you will need

pencil sharpener (an exacto knife will do if a
sharpener is unavailable)

wire strippers or scissors, if the wires are
insulated.

Procedure

Remove the erasers and their metal sleeves from
both pencils, and sharpen both ends of both pencils.

Fill the glass with warm water.

Attach wires to the electrodes on the solar cell
or battery, and the other ends to the tips of the pencils, as shown in the
diagram above. It is important to make good contact with the graphite in the
pencils. Secure the wires with tape.

Punch small holes in the cardboard, and push the
pencils through the holes, as shown in the diagram above.

Place the exposed tips of the pencils in the
water, such that the tips are fully submerged but are not touching the
bottom, and adjust the cardboard to hold the pencils.

Wait for a minute or so: Small bubbles should
soon form on the tips of the pencils. Hydrogen bubbles will form on one tip
(associated with the negative battery terminal - the cathode) and oxygen
from the other.

Specific things you can point out:

It is very important to note that electrolysis
does not depend intrinsically on the generation of heat (although some may
be produced, for example, from the turbulence created by the bubbles of gas
in the liquid). Therefore, it is not subject to a fundamental thermodynamic
limitation on efficiency, which would be the case if a fixed fraction of the
energy used was converted into heat (since creating heat creates entropy).
Therefore, electrolysis can be (and is) performed at very high efficiencies
close to 100%.

If you use a battery, then chances are that the
battery was charged with electricity produced by burning fossil fuels, so
that the hydrogen you produce isn't produced cleanly. If you use a solar
cell, however, then the hydrogen will be produced cleanly, except for any
pollutants that were emitted when the cell was made (we say that the solar
cell has no "point-of-use" emissions).

If you use a battery or solar panel that
generates less than 1.5 volts, then it will be necessary to add an electrolyte,
such as a salt, acid, or base, that will disassociate into charged ions
and increase the flow of electrical current.

We use pencils as electrodes because the carbon
(in the form of graphite) that they consist of will not dissolve into the
water under the influence of the electron current - the carbon is
electrically neutral.

If the electrodes are made of metal, and
if there is another metal dissolved into the water, then the metal electrode
will become plated with the dissolved metal. This process is called electroplating,
and is used in industry to produce aluminum and also to plate things with
gold or silver.

Advanced Experimentation

Advanced students may want to study the efficiency
of the electrolysis project. This can be done, under careful supervision (since
you will be collecting hydrogen), in the following way:

First make the following measurements carefully
and simultaneously:

Collect the hydrogen produced with a test
tube: The test tube should be initially filled with water (by
submerging it) and positioned over the negative electrode, with the open
end submerged and the closed end pointing upwards (such that the tube is
completely filled with water at the start of the experiment). Run the
experiment until the water level inside the test tube matches the water
surface level. At this point the pressure of the hydrogen will equal
ambient pressure. Stop the experiment when this level is reached.

Measure the current I
in amps: Do
this by placing an ammeter in the electrolysis circuit - have someone
read the meter during the experiment to get a good idea of the average
current. Make sure you express the result in amps, which may require
conversion from milliamps.

Time the entire experiment with a
stopwatch in seconds. (This may be a large number).

Measure the ambient (room) temperature in
Celsius degrees.

Calculate the volume of hydrogen produced at
ambient pressure in cubic meters: Measure the dimensions of the test
tube, and the length of the tube above water. Make sure you answer is
expressed in cubic meters. For example, if you initially calculate the
volume in cubic centimeters, divide your answer by 1 million.

Now calculate the theoretical (maximum) volume
of the hydrogen produced, also in cubic meters, from the other data for the
current and the time, using "Faraday's First Law":

Finally, calculate the efficiency by comparing
the volume produced to the theoretical maximum volume:

Efficiency (in %) = 100 x Vproduced /
Vtheoretical .

Discuss the possible sources of
inefficiencies/errors, such as

Failure to capture all the hydrogen

Energy lost to heat

Various measurement errors

How Does it Work?

The chemical equation for electrolysis is:

energy (electricity) + 2 H2O
-> O2 + 2 H2 .

At the cathode (the negative electrode), there is a
negative charge created by the battery. This means that there is an electrical
pressure to push electrons into the water at this end. At the anode (the
positive electrode), there is a positive charge, so that electrode would like to
absorb electrons. But the water isn't a very good conductor. Instead, in order
for there to be a flow of charge all the way around the circuit, water molecules
near the cathode are split up into a positively charged hydrogen ion, which is
symbolized as H+ in the diagram above (this is just the
hydrogen atom without its electron, i.e. the nucleus of the hydrogen atom, which
is just a single proton), and a negatively charged "hydroxide" ion,
symbolized OH-:

H2O -> H+ + OH- .

You might have expected that H2O would
break up into an H and an OH (the same atoms but with neutral charges) instead,
but this doesn't happen because the oxygen atom more strongly attracts the
electron from the H - it steals it (we say the oxygen atom is more
"electronegative" than hydrogen). This theft allows the resulting
hydroxide ion to have a completely filled outer shell, making it more stable.

But the H+, which is just a naked
proton, is now free to pick up an electron (symbolized e-) from the
cathode, which is trying hard to donate electrons, and become a regular, neutral
hydrogen atom:

H+ + e- -> H

This hydrogen atom meets another hydrogen atom and
forms a hydrogen gas molecule:

H + H -> H2,

and this molecule bubbles to the surface, and wa-la!
We have hydrogen gas!

Meanwhile, the positive anode has caused the
negatively charged hydroxide ion (OH-) to travel across the container
to the anode. When it gets to the anode, the anode removes the extra electron
that the hydroxide stole from the hydrogen atom earlier, and the hydroxide ion
then recombines with three other hydroxide molecules to form 1 molecule of
oxygen and 2 molecules of water:

4 OH- _> O2+ 2H2O + 4e-

The oxygen molecule is very stable, and bubbles to
the surface.

In this way, a closed circuit is created, involving
negatively charged particles - electrons in the wire, hydroxide ions in the
water. The energy delivered by the battery is stored by the production of
hydrogen.

Water is perhaps the most important substance to
life on Earth. It is a simple compound made from the two elements hydrogen (H)
and oxygen (O), and each molecule of water consists of two hydrogen atoms and
one oxygen atom. Thus we write the chemical formula for water as "H2O".

Hydrogen itself is also a very important element in
the universe. For example, it is the fuel for the Sun, which generates power by
fusing (combining) hydrogen atoms into a helium in a process call nuclear
fusion. Because it can be obtained from water, as this project demonstrates, the
German's call hydrogen "wasserstoff", which literally means
"water stuff".

Suppose that you just happen to have some pure
hydrogen gas on hand, stored in a container. The hydrogen gas consists of H2
molecules zipping around in a container (hydrogen atoms like to bond together
into H2 molecules). If there also happens to be oxygen gas around (O2),
and there is always plenty oxygen in the air (air consists of about 20%
oxygen), then the oxygen can react violently with the hydrogen gas, such that
the hydrogen burns, or combusts, with the oxygen to form water and
heat, according to the chemical reaction

2H2 + O2->
2 H2O + energy (heat).

Therefore, if you have some hydrogen, you can burn
it for fuel to generate heat!

Generating heat, however, is not always the best
thing to do, because entropy, which may be thought of as molecular
disorder, is created when heat is generated, and that can limit
the efficiency of devices that use that heat energy to do useful work (For more
info on entropy, see the section on entropy
in our Energy Physics Primer). Fortunately, there exists a device called a fuel
cell, which can chemically combine hydrogen with oxygen to make
electricity. After you complete this project, you may want to also want to cover
our project on exploring
fuel cells.

Fuel cells can also accomplish what this project
demonstrates - electrolysis - which generates hydrogen from water.

Burning fossil fuels on the other hand, always
results in carbon monoxide (CO) and/or carbon dioxide (CO2), which is
produced when the carbon atoms combine with oxygen. These compounds are now
considered pollutants because they are greenhouse gases - that is, they help
trap heat near the Earth's surface, causing the Earth's surface temperature to
rise, i.e. global
warming.

Burning fossil fuels such as coal also usually
releases other pollutants, including sulfur dioxide, mercury, and uranium to the
atmosphere, because these substances are usually present to varying extent in
fossil fuels. But if we can obtain hydrogen without producing greenhouse gases
or these other pollutants, then hydrogen is a better fuel to use than fossil
fuels. Many people are now hopeful that a "hydrogen-economy"
will soon replace our fossil fuel economy.

Obstacles to a hydrogen-economy

There are two obstacles to a hydrogen-economy.

It takes alot of volume (or energy) to store
hydrogen - usually five times or so the volume, at reasonable pressures,
needed to store an equivalent amount of energy with gasoline. One
company that has made headway on solving this problem, however, is Dynetek (www.dyneteck.com).

There is no hydrogen infrastructure:
Making the transition to a hydrogen economy might mean having to scrap the
fossil fuel infrastructure that we have already developed. One company that
has made progress on refueling equipment is Stuart Energy (www.stuartenergy.com).

Both of these problems might be surmounted by using
synthetic fuels. For example, it is possible, using a catalyst, to
combine water, carbon dioxide (extracted from the air), and renewable
electricity to make fuels such as methanol, a carbon-based fuel. When this fuel
is burned, water and carbon dioxide are produced. But because the carbon dioxide
used initially to make the fuel was extracted from the air, the cycle is closed
with respect to both water and carbon dioxide, and so won't contribute excess
carbon dioxide to the atmosphere. Fuel cells can already use such fuels
(either by extracting the hydrogen from the fuel prior to the fuel cell, or even
directly in certain types of fuel cells).

Is Hydrogen Dangerous?

Some people are worried that hydrogen might be too
dangerous. It is true that hydrogen is a very explosive fuel, but so is natural
gas and gasoline. For example, movies commonly depict automobiles burning up
after crashing, and explosions involving natural gas are reported in the press
from time to time. Two famous disasters involving hydrogen are the explosion of
a zeppelin (an airship) called the Hindenburg (in 1937), and the explosion of
the Space Shuttle Challenger (in 1986). You may want to study these disasters as
a class project. The Hindenburg explosion, although often cited as an example of
the danger of hydrogen, is thought by many to have been caused by flammable
paint that caught fire from an electrical spark, and so might have caught fire
even if the zeppelin had been filled with helium (an inert, nonflammable gas).
Moreover, most of the people that died may have done so from coming into contact
with burning diesel fuel (which powered the Hindenburg's airplane-type
prop-engines) or from jumping of the Zeppelin before it landed.